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is some 16 or 18 times that of water at the same pressure, and
the jet of steam escaping from D is so suddenly cooled by the
tank water through c, that it has not time to reduce its velocity
to that due to it as water, and therefore succeeds in piercing the
boiler water, carrying the tank water with it. An overflow takes
place at F when first starting, which, however, ceases when cocks
c and H have been mutually adjusted.

A lifting injector must permit of regulation at the orifice D
and ring orifice E, for the conditions of vacuum-forming and
water-forcing are quite different, and the former must be first
satisfied, after which the latter may be met without disruption
of the water column, An ingenious method of automatic lifting
injector is now in operation, where the throat E is split longitu-
dinally, and one half hinged near the annulus, the' flap nozzle' thus
formed also causing are-starting, should the fluid tend to disunite,

Slightly altering the cone proportions, and giving the water a
few feet of head, produces an injector workable by exhaust steam ;
but, on account of the variation in pressure, the flap nozzle must
be provided.

Other Mountings for the boiler are: a blow-off cock near the
firebox bottom, gauge cocks about 3 ins. above and below the
water line, fire bars and bearers, furnace doors, rnud plugs, fusible
plug in furnace crown to melt in case of overheating, and thus
•cause the fire to be extinguished, clack box (j, Fig. 682), damper
for regulating draught, a filling branch when no other hole is
-convenient, and sometimes a scum cock. (SeeApp. •//.,/. 902.)

Combustion.—Combustion or burning is rapid chemical
combination accompanied by heat and sometimes light. If con-
siderable noise be caused it is termed an explosion. During
•combination, heat is produced equal to that required to separate
the same elements. The separation of carbon and hydrogen,
and their re-combination with oxygen, is what the engineer
needs to understand, so \ve will consider the burning of a simple
hydrocarbon like marsh gas, shewn by the formula :

Marsh gas 4- Oxygen. = Carbon dioxide -t- Water (steam)



a case of complete combustion, for no single element remains.